1. Field of the Invention
The present invention relates to systems and methods for holographic recording and readout of data, and in particular, to write verification of holographically written data. The present invention further relates to systems and methods for implementing additional optical processing functions, such as optical pre and/or post-processing steps required in certain holographic storage media.
2. Background Art
Holographic data storage (“HDS”) is a form of optical data storage, whereby recording of data is achieved by illuminating a photosensitive medium with intersecting reference and data light beams. The spatial modulation of light intensity produced by interference of the beams is recorded in a holographic data storage medium by modification of the dielectric properties of the medium, either in the form of periodic spatial modulation of the refractive index of the medium or of the absorption of the medium, to constitute a grating or a hologram. One form of holographic data storage sequentially records localized holograms each containing a single bit, and is sometimes termed micro holographic bit storage. Another form of holographic data storage comprises volumetric page holographic recording which allows a large amount of data to be recorded in parallel in the form of a 2 dimensional bit array or data page. This is accomplished by placing a spatial light modulator in the optical path of the data light beam. The spatial light modulator imparts a data page on the data light beam by modulating its spatial profile.
It is also well known in the prior art that multiple data holograms can be recorded within a same recording volume by means of one of a plurality of multiplexing techniques. The techniques generally involve changing one of the properties of the reference beam used to record each data hologram such that the data can be selectively retrieved only by illumination of its data storage location in the holographic data storage medium by its associated reference beam. Detection is typically performed by imaging the optical data signal upon a suitable photodetector, which may comprise a single photodetector for detecting single bit signal or a two dimensional photodetector array for detecting a two dimensional data page.
As in all digital storage methods, holographic data storage typically requires methods for write verification. In particular, such methods of verifying holographically written data must minimize degradation of storage capacity and data rate while ideally providing bit-level verification and also enabling maximum usage of existing optical elements and laser power sources. Data readout after writing accomplishes bit-level data verification, but this method halves the effective data write rate. Other methods have been proposed for write verification of holographically written data, but suffer from one or more disadvantages. For example, U.S. Pat. No. 6,788,443 discloses the use of associative readout by illumination with a single data beam of all the multiplexed holograms, which each simultaneously produce a diffracted signal along the path of its recording reference beam proportional in amplitude to a spatially integrated correlation product between the readout and recorded data pattern. The “associative readout” method can thus simultaneously readout all co-located data holograms: but requires additional detection and signal processing means. Furthermore, in the case of data page holograms, detection of diffracted signal intensities does not allow actual bit-level data verification due to the spatial integration process, but rather provides an average signal to noise ratio, which in the case of intra-page non uniform degradation effects is not an adequate measure of worst case bit error rate. In another prior art example, U.S. Pat. No. 6,956,681 discloses the use of a polarization offset, of recording reference beam with respect to a polarization of recording data beam during writing. Orthogonal polarization component of reference beams does not interfere with recording data beams, but diffracts off of data hologram being recorded. Subsequent detection of the offset polarized diffracted data beam provides a means for real time monitoring of data diffraction efficiency, but diverts a portion of the total recording power for readout, and also requires additional polarization separation and detection means.
Another significant problem of real time monitoring (reading) during data recording is that the media's photoactive species responsible for recording are equally affected by any additional optical exposure for readout. Thus, any data readout process intervening between successive data recording sequences effectively reduces the number of remaining species available for recording and, correspondingly, the photosensitivity of the medium with regard to subsequent data recording, thus degrading both the effective data write transfer rate, as well as the total storage capacity of the media.
Holographic data storage systems may employ several different configurations for recording and readout of data hologram, such as transmissive or reflective hologram recording geometries with recording beams incident upon same and opposite sides of the holographic data storage medium, respectively. Holographic data readout may be achieved using a reference beam identical to recording reference beam. Another prior art holographic data readout configuration comprises a phase conjugate readout arrangement, whereby a reference beam generally identical to, but propagating in opposite direction to recording reference beam is employed. In this case reconstructed data signal is diffracted along the same path as but in opposite direction to recording data beam, thus resulting in a more compact system. Further background information on holographic storage may be found in Geoffrey W. Burr, Holographic Storage, Encyclopedia of Optical Engineering, Marcel Dekker, Inc., 2003, and references contained therein. Based on the aforementioned limitations of the existing art, improved methods and systems for verifying holographically recorded data are needed, that can further reduce degradation of storage capacity and effective write data rate, provide bit level data verification and also enable maximum usage of existing optical elements and laser power sources.